The PCV valve is an emission control device operated by intake manifold vacuum to prevent crankcase blow-by gases from escaping into the atmosphere. When the engine is running at idle, the valve opens slightly to allow a small amount of blow-by gas to be drawn into the intake manifold/dynamic chamber where it mixes with intake air for combustion in the cylinders. As engine speed increases, the valve opens to allow more of the blow-by gases into the chamber.